linux-kernel - Re: [PATCH RFC 0/7] KVM: nVMX: enlightened VMCS initial implementation

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Date:   Thu, 21 Dec 2017 15:32:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org
Subject: Re: [PATCH RFC 0/7] KVM: nVMX: enlightened VMCS initial
 implementation

On 21/12/2017 13:50, Vitaly Kuznetsov wrote:
> I'm back with (somewhat frustrating) results (E5-2603):

v4 (that would be Broadwell)?

> 1) Windows on Hyper-V (no nesting): 1350 cycles
> 
> 2) Windows on Hyper-V on Hyper-V: 8600
> 
> 3) Windows on KVM (no nesting): 1150  cycles
> 
> 4) Windows on Hyper-V on KVM (no enlightened VMCS): 18200
> 
> 5) Windows on Hyper-V on KVM (enlightened VMCS): 17100

What version were you using for KVM?  There are quite a few nested virt
optimizations in kvm/queue (which may make enlightened VMCS both more or
less efficient).

In particular, with latest kvm/queue you could try tracing vmread and
vmwrite vmexits, and see if you get any.  If you do, that might be an
easy few hundred cycles savings.

Paolo